ARM: at91: at91sam9260: add missing pinctrl-names on mmc

pinctrl-names was missing causing mmc pinctrl to never be requested.

Signed-off-by: Jean-Christophe PLAGNIOL-VILLARD <plagnioj@jcrosoft.com>
Acked-by: Ludovic Desroches <ludovic.desroches@atmel.com>
[nicolas.ferre@atmel.com: added a commit message taken from Ludovic]
Signed-off-by: Nicolas Ferre <nicolas.ferre@atmel.com>